Will Genia, the wonderful Wallaby half-back with the all-seeing eye, spotted something amid the fire and fury of last week’s Test in Brisbane that told him everything he needed to know about Jonathan Sexton, just about the most important individual in a Lions team now standing on the brink of a something too rare to be dismissed as merely special: a series victory in the southern hemisphere.

It was nothing so delicate as a perfectly weighted pass or clever little chip into space, still less a barely perceptible readjustment of balance that had a marauding back-row forward clutching filthy great handfuls of thin air instead of grabbing his man by the short and curlies. What impressed Genia, right down to his marrow, was the way Sexton tore strips off his own captain, the Welsh flanker Sam Warburton, for daring to stand in the wrong place.

“He has some composure about him, but he’s also decisive with his players,” the Australian said yesterday. “He knows what he wants them to do and he tells them. There was a moment in last Saturday’s game when he seemed to be getting stuck into Warburton because he wasn’t where he should have been.” Genia might have added that if the Wallabies had a game manager of the same quality at outside-half, instead of one who stays out until 4am in the middle of a Test week, they might not be 1-0 down with two to play. But that’s another story.

When Sexton feels the need to go off-piste, a late-night feed at the kind of downtown fast-food joint frequented by the errant James O’Connor is not his first choice, or even his 101st. At home in Ireland, he has been known to leave the bustle of Dublin for a quiet spell at a favourite spot in County Kerry, spending hours on the cliffs near Ballybunion, where he holidayed as a kid. Here in Australia, where solitude is not an option for anyone with the remotest connection to the Lions, he disappears into himself as a big game approaches, re-emerging into view only when he knows he must, shortly before kick-off.

“All the talk about how big this game is… it gets the nerves going,” he said. “But it’s normal enough for me to be nervous at this stage of the week. There’s something about the really important occasions that makes you feel different. It starts the day before the match and my way of dealing it is to go quiet for a while. I try not to talk about what’s coming up with anyone until we’re maybe four hours away. Once I’m out on the pitch it always seems like any other game of rugby, even if the potential rewards are greater. But yes, I do have to control my nerves during the build-up.

“There’s an interesting mix in the squad right now. There are guys who played for the midweek team against the Rebels who are not directly involved in this game and are a million miles away from where we are at the moment. So you have some people relaxing a little and others who are torn up with nerves. The pressure on us is really kicking in.”

Sexton’s reward for absorbing that pressure and getting it right today will be rich indeed, not least in the historical sense. No Irish outside-half has ever piloted the Lions to a series victory: not Tony Ward, not Ollie Campbell… not even the great Jackie Kyle, who tried everything he knew – and he knew as much as anyone who ever played the game – to beat the All Blacks in 1950, yet left New Zealand with nothing to show for it but a single, solitary draw in damp, dank Dunedin. If Sexton can strike gold here in Melbourne, he will join Barry John and Phil Bennett of Wales, Rob Andrew of England and Gregor Townsend of Scotland as a winning No 10.

The 27-year-old Leinsterman – he turns 28 five days after the completion of this tour – understands all this: the value of securing a seat in the pantheon is not lost on him. “As a player, you want to be talked about in years to come,” he acknowledged. “This is our chance to make sure that happens. And we have to take the chance while it’s there: certainly, we don’t have the luxury of thinking ‘if we lose, there’s still Sydney next week’. If we win this game, we achieve what we set out to achieve. Should the Wallabies win it, all the momentum will be with them.

“Brian O’Driscoll [the four-time Lion who has spent the last few years operating alongside Sexton in the Leinster midfield] has been the one putting this into the correct perspective. He spoke to us about coming here on his first tour in 2001 and thinking he’d have other chances to win a series, and making the point that in actual fact, the chances don’t come around too often. He’s made us realise that we need to grab it while it’s there – not just for ourselves as individual players, but also for people like Brian and Paul O’Connell, guys who deserve it most after all they’ve done for the sport.

“Paul is injured and out of the series now, but he’s still playing such a big part. I haven’t been in any of the forwards’ meetings, where I’m sure he’s been contributing a lot, but even for us backs, he’s a major presence – a very important figure. He’s not going to be out there on the pitch with us, but having him around has been great. In a way, it doesn’t feel like we’ve lost him at all.”

According to Rob Howley, the Lions’ attack coach, Sexton is “very composed, a natural navigator and extremely open to new ideas”. The former scrum-half also believes he has forged a mature understanding with O’Driscoll from which his approach to game management has evolved. “Jonny likes to play,” Howley said, “and there are times when he tries to play in areas of the field or in match situations that are not absolutely ideal. You can see Brian communicating with him, sometimes with just a look. It’s a ‘not here, not now’ message, and Jonny responds to it. You don’t always see that in a top-class international No 10, do you?”

What is more, Sexton has responded more positively than he might have done to the loss of the goal-kicking job – a task he confidently expected to perform when he was selected for this tour back in April. Neil Jenkins, the kicking coach, mentioned earlier this week that the Irishman was “disappointed” with the situation and there was a moment yesterday when he let it show. “I’m finding it different,” he confessed when asked about being beaten to the tee by the Welsh full-back Leigh Halfpenny. “I’ve worked all year with this in the back of my mind and it hasn’t happened. Whenever I’ve been on the pitch, Leigh has been there too – and his stats speak for themselves. You can’t argue with them. It’s the same as Owen Farrell [Sexton’s understudy at outside-half]. He’s been hitting the ball really well.

“Since the last World Cup, when I had a bit of a blip, I’ve been happy with my kicking, and I’m continuing to work on it here in case Leigh goes down or has a bad day. I’d like to do the job, obviously: it’s something I’ve done all my life and not doing it is strange. But it means there’s a little less in the way of pressure. I can enjoy that, for sure.”

All things considered, the Lions are lucky to have Sexton. His innate rugby intelligence has allowed him to build a prosperous partnership with his fellow inside back Jonathan Davies, even though the resourceful Welshman is more suited to the outside centre position, and the coaches expect him to do the same today with the England scrum-half Ben Youngs, with whom he has spent precious little time in a match situation.

“We played together a little bit at the end of the game against Queensland Reds,” he remarked, calm as you like, “and we’ve trained all week. We’re pretty much on the same wavelength.” He might just as well have shrugged and asked: “Why all the fuss?”

Many moons ago, the aforementioned Barry John cut short a technical discussion about passing trajectories with Gareth Edwards by uttering one of rugby’s most celebrated one-liners. “You throw it, Gar, and I’ll catch it,” he said. There will probably be a bit more to it when the Wallabies come rampaging at the red-shirted half-backs today, but if Sexton can bring the best of himself to the arena, a 33-minute “get to know you” session with Youngs in Queensland and a handful of training sessions may just be enough to cement another winning Lions half-back axis.

Sexton statistics: fly-half in figures

36 Tests played by Sexton for Ireland – making 28 starts – after 11 appearances for Ireland A

282 Points scored by the 27-year-old in Test matches for Ireland, since his debut four years ago

3 Number of Heineken Cups Sexton has won with Leinster – in 2008-09, 2010-11 and 2011-12

110 Matches played by Sexton in seven years at Leinster – scoring 16 tries and 1,027 points overall